1. Field of the Invention
This invention relates to downhole tools useful for forming boreholes into the earth. Specifically, a two stage downhole drilling fluid filter for these downhole tools is disclosed that is resistant to clogging by lost circulation material and detritus.
2. Description of the Related Art
When drilling boreholes into the earth, a liquid drilling fluid, now well known simply as xe2x80x9cmudxe2x80x9d or xe2x80x9cdrilling mudxe2x80x9d, is often used to flush the cuttings from the bottom of the well bore to the surface. Originally, the mud was used only for flushing out the cuttings. It was not long however, before the drilling industry realized that the drilling mud, often supplied at high pressures and high flow rates, could be used to power other devices in the drill string that support the drilling operation, including telemetry pressure pulses, power, and primary well control.
However, at times during drilling, a portion of the drilling fluid may flow into the formation being drilled. This is considered a serious situation, and oftentimes special additives called lost circulation materials (LCM) are added to the mud to slow or stop this undesired diversion of the mud. LCM is designed to plug the types of gaps in the rock formations that tend to open when circulation is lost. Unfortunately, these gaps are very similar to the clearances and passageways in the drilling mud powered tools. Consequently, the designers of these tools place limits on how much and what types of LCM can be used with their tools.
Another problem with the use of these drilling fluid using downhole tools is that, at times, other undesirable materials that damage the tools find their way into drilling mud. Items such as plastic wrapping and bagging material and other contaminants introduced by the field personnel can contaminate the drilling mud and block the fluid passages in the mud powered tools as badly as LCM.
It is now commonplace to have numerous tools in the drilling string which use the drilling mud to supply power for their operation. Such tools include drill bits, drilling motors, drilling turbines, rotary directional drilling devices, mud driven electric generators, hole opening devices, measuring while drilling tools, downhole communication devices, and many others.
In many of these tools, the mud powered systems are designed to tolerate these particles by allowing very high volume flow through the system and by providing large restrictions (chokes) when it is necessary to provide a pressure differential.
In other tools, particularly rotary drilling tools, single stage filter elements have been used. The total filter area in these tools is sized in a manner to provide sufficient flow through the filter if the filter gets partially obstructed and blocked by particles. Unfortunately, these filters can collapse under the differential pressure once a sufficiently high number of holes are blocked.
In addition, these filters tend to exhibit uneven wear. After long use, single stage filters tend to wear preferentially at the inlet end. Typically only the first 10% to 50% of the xe2x80x9cupstreamxe2x80x9d end of the filter wears out, leaving the majority on the surface unworn. In some cases this uneven wear forces the entire fluid using tool to be rebuilt when only a portion of the filter erodes away.
Newer types of rotary drilling tools may have drilling fluid powered actuators that have relatively small passageways leading from rotary vales and have fluid chokes to create working pressure differentials in the drilling fluid, as described in U.S. Pat. Nos. 5,265,682; 5,553,678; 5,803,185; 6,089,332; 5,695,01 5; 5,685,379; 5,706,905; 5,553,679; 5,673,763; 5,520,255; 5,603,385; 5,582,259; 5,778,992; 5,971,085 all herein incorporated by reference. In these tools, larger particulates present in the drilling fluid in form of drill cuttings or drilling fluid additives can block the choke holes in the actuation system or cause damage to or jamming of the rotary valve. In particular, high levels of lost circulation material added under certain operating conditions can adversely affect the actuation system.
Therefore, some form of filtering is required in these tools, as these particulates must be filtered from the drilling fluid diverted from the main fluid flow for the hydraulic actuation system. The filter also needs to be kept clean during operation to ensure functionality of the actuators and prevent collapse of the filter element due to a build-up in differential pressure when filter holes get blocked.
Unfortunately, the hereinbefore-described limitations of the single stage filter have affected the performance of these devices. For example, due to space and structural constraints, the prior art filters had relatively small holes for fluid flow. The small hole size limits space availability in the tool and requires the filter element to be a main structural component in the tool. Additionally, the filter hole size and shape was limited to prevent early blockage of the filter element. These constraints became particularly limiting when attempts were made to scale these tools down to smaller borehole diameters.
Disclosed is a two stage filter for a downhole tool for filtering solid materials from the drilling fluid. The downhole tool may be any of the type using the drilling fluid for operations, but the two stage filter is particularly applicable to rotary steerable type downhole tools. The two stage filter comprises a first, outer filter section and a second, inner filter section, the drilling fluid flowing from the first section to the second section. The flow area of the first filter section is greater than the flow area of the second filter section.
In this tool, a majority of the drilling fluid flowing through the second filter section is received from the first filter section. The two filter sections of the tool have holes or apertures in them. The average cross-section area for the apertures in the inner section may be greater than an average cross-section area for the apertures in the outer section.
In this tool the average cross-section area for the second plurality of apertures may be more than 20% greater than the average cross-section area for the first plurality of apertures. Also, the flow area of the first filter section of the tool may be at least two times greater than the flow area of the second filter section.